Relatively small (e.g., 3 to 5 IRE) signals encoding digital information can be admixed together with composite video signals without being readily evident in television pictures generated from those composite video signals if suitable restrictions on the digital signal format are observed. A representative system for doing this is described by Jian Yang in his U.S. patent application Ser. No. 08/141,070, filed Oct. 26, 1993, entitled APPARATUS FOR PROCESSING NTSC TV SIGNALS HAVING DIGITAL SIGNALS ON QUADRATURE-PHASE VIDEO CARRIER and incorporated herein by reference. Yang describes binary phase-shift-keyed (BPSK) modulation of a suppressed carrier that is the same frequency as a video carrier and is in quadrature phasing therewith. Yang advocates the BPSK signals being constrained to about 2 MHz bandwidth so as to avoid crosstalk into chroma in TV receivers that separate chroma from luma without recourse to comb filtering. Yang indicates a preference for passing the data to be transmitted through a partial-response filter for increasing its correlation at corresponding points along successive horizontal scan lines in the composite video signal, this providing a basis for using line-comb filtering in the digital signal receiver to separate PSK subcarrier from the luminance portion of the composite video signal. Yang also advocates repeating frames of the BPSK in antiphase in successive pairs of consecutive frames of the NTSC television signal. Such repetition of data in pairs of frames makes the BPSK accompanying the composite video signal detected from the NTSC television signal less apparent in images that are generated from the composite video signal for viewing on a screen. Such repetition of data in pairs of frames also provides a basis for using frame-comb filtering in the digital signal receiver to separate the BPSK from the luminance portion of the composite video signal that describes static portions of successive television images.
Receivers for the Yang system are also described by Thomas Vincent Bolger in his U.S. patent application Ser. No. 08/141,071, filed Oct. 26, 1993, entitled RECEIVER WITH OVERSAMPLING ANALOG-TO-DIGITAL CONVERSION FOR DIGITAL SIGNALS WITHIN TV SIGNALS and incorporated herein by reference. These receivers digitize the response of a quadrature-phase video detector using an oversampling analog-to-digital converter. The digitized quadrature-phase video detector response is subjected to digital frame-comb and line-comb filtering to suppress remnant composite video signals; the comb filtering response is supplied to multi-level decision circuitry to recover bit-serial digital data transmitted by the BPSK; and the bit-serial digital data is supplied to a decoder that corrects the digital information in the data using forward-error-correcting codes contained therein. The inventions described by J. Yang and T. V. Bolger in their patent applications, like the inventions described herein, are assigned to Samsung Electronics Co., Ltd., pursuant to pre-existing employee agreement so to assign inventions made within the scope of employment.
Constraining the BPSK signals to about 2 MHz bandwidth, so as to avoid crosstalk into chroma in TV receivers that separate chroma from luma without recourse to comb filtering, reduces the average uncorrected data rate to about 2 megabits per second in the preferred Yang system. Average uncorrected data rate is lowered in the preferred Yang system because frames of the BPSK are transmitted once in positive logic and once in negative logic, in successive pairs of consecutive frames of the NTSC television signal. Certain data requires more than this 1 MHz average bandwidth. Stereophonic digital audio of "compact disk quality" requires about twice this bandwidth, by way of example.
By simply modifying the preferred Yang system, so as not to transmit each successive new frame of the BPSK twice, the average uncorrected data rate can be doubled. With regard to video transmission and reception, this simple modification requires sacrificing the frame-averaging effect associated with lags in the responses of the kinescope phosphors and the retinal system of a human viewer that suppress the frame-rate flicker perceived by that human. With regard to data transmission and reception, this simple modification requires sacrificing the advantage of being able to use highpass frame-comb filtering to suppress the portions of the luminance signal which accompanies the BPSK that change between adjacent lines but not from frame to frame. These portions of the luminance signal jam the proper detection of the BPSK from time to time and cause errors in the detected BPSK signal.
Two different types of partial-response filtering are of interest in regard to the inventions disclosed in this specification and its drawing. A partial-response filter that will be referred to as a "pre-line-comb" partial-response filter is composed of one or more sections, each of which sections includes a respective two-input exclusive-OR (XOR) gate having a first input for receiving serial-bit data for partial-response filtering and an output from which section response is taken. Each section further includes a "1-H" digital delay line for applying the section response, as delayed by "1-H" which is the duration of one horizontal scan line in an NTSC television signal, to the second input of the XOR gate in that section. A partial-response filter that will be referred to as a "pre-frame-comb" partial-response filter is composed of one or more sections, each of which sections includes a respective two-input exclusive-OR (XOR) gate having a first input for receiving serial-bit data for partial-response filtering and an output from which section response is taken. Each section further includes a "1-F" digital delay line for applying the section response, as delayed by "1-F" which is the duration of one frame of NTSC television signal, to the second input of the XOR gate in that section. Digital delay lines as used in these filters are commonly constructed using random-access memories (RAMs) arranged for operation in a read-then-write-over mode while being addressed by an address counter counting the number of samples per line in the case of a "1-H" digital delay line or counting the number of samples per frame in the case of a "1-F" digital delay line.
In the Yang system "pre-line-comb" partial-response filtering of the serial-bit data is done at the BPSK digital signal transmitter to complement line-comb filtering done at the digital signal receiver, and in a modification of the Yang system made in accordance with one embodiment of the invention disclosed herein this "pre-line-comb" partial-response filtering is retained. It is augmented by further "pre-frame-comb" partial-response filtering of the serial-bit data being done at the BPSK digital signal transmitter to complement frame-comb filtering done at the digital signal receiver. This frame-comb filtering can comprise highpass frame-comb filtering to suppress the portions of the luminance signal which accompanies the BPSK that change between adjacent lines but not from frame to frame. This highpass frame-comb filtering will provide a somewhat different result because data frames are not transmitted twice during each of successive pairs of consecutive frames of the NTSC television signal, once in positive logic and once in negative logic. The frame-comb filter response will have more signal levels to be separated from each other by the symbol decision circuitry.
Performing "pre-frame-comb" partial-response filtering of the serial-bit data on a continuous basis, rather than transmitting frames of the BPSK twice, theoretically results in the full baseband below chroma being available on average for the transmission of BPSK, rather than half that bandwidth. At the same time there is no need to sacrifice selectivity against the portions of the luminance signal which accompanies the BPSK that change between adjacent lines but not from frame to frame. So, the jamming capability of the video signal remnants to interfere with BPSK detection is reduced.
In the practical world "pre-frame-comb" partial-response filtering of the serial-bit data cannot be carried out on an altogether continuous basis. This is because the data stored in the frame store of the "pre-frame-comb" partial-response filter in the digital signal transmitter and the data stored in the frame store of the "pre-frame-comb" filter of in the digital signal receiver must be synchronized from time to time. Synchronization is implemented in accordance with an aspect of the invention by discarding the contents of both frame stores at the beginnings of recurrent start frames, during each of which start frames a known null pattern is transmitted. This null pattern can be composed of consecutive 1-H intervals of alternated ZEROs and ONEs, with the bit pattern reversed from each 1-H interval to the next, by way of example. Synchronization could be done every eighth frame in response to the ghost cancellation reference signal accompanying the NTSC television signal being in a particular one of the states it cyclically assumes every eighth frame. Synchronizing the digital signal transmitter and receiver every eight frame reduces average data rate by 12.5% from that theoretically achievable. Synchronization is done primarily to accommodate the tuner of the digital signal receiver changing between channels transmitting digital data, however. So synchronization need be done only frequently enough that a person doing the channel switching can ascertain within a reasonably short time whether or not he has tuned to a channel he desires to be received. Synchronization every sixty-fourth frame is probably frequent enough, which reduces average data rate about 1.5% from that theoretically achievable. Alternatively, synchronization every thirty-second frame reduces average data rate about 3% from that theoretically achievable.